Non-aqueous electrolyte batteries, which typically include lithium ion secondary batteries, have recently been and currently are finding practical applications as electrochemical devices showing a high energy density. Ordinary lithium ion secondary batteries have a positive electrode formed by using transition metal oxide as positive active material and a negative electrode formed by using carbon as negative active material and contains an electrolyte salt which is a lithium salt such as LiPF6 and a non-aqueous electrolyte which is a carbonate type organic solvent. However, intercalation/deintercalation of most lithium ions takes place at a potential level lower than the reduction decomposition potential of the non-aqueous solvent to the carbon material. Therefore, while lithium ion secondary batteries show a high energy density, they are accompanied by a problem of a short service life and poor operation characteristics at high temperature.
Proposals have been made to use a negative active material with which intercalation/deintercalation of lithium ions takes place at a potential higher level than the reduction decomposition potential of the non-aqueous solvent. According to these proposals, lithium titanate with which intercalation/deintercalation of lithium ions takes place at about 1.5 V is employed as the negative active material (see, for example, Patent Document 1). Non-aqueous electrolyte batteries employing lithium titanate as negative active material shows an excellent service life and good operation characteristics at high temperature. Many papers have been published and many reports have been made at scientific conferences on non-aqueous electrolyte batteries using lithium titanate as negative active material and such non-aqueous electrolyte batteries have been commercialized as memory backup (see, for example, Non-Patent Document 1, coin-type lithium ion secondary battery (Sony) etc.).    Patent Document 1: JP-B-3502118    Non-Patent Document 1: Journal of Power Sources 146 (2005) 636-639
However, Non-aqueous electrolyte batteries employing lithium titanate as negative active material are accompanied by a problem of gas generation. This problem hardly occurs in coil-type batteries and cylindrical batteries having a very strong battery case can but can arise as swollen batteries in the case of flat type batteries and batteries using metal resin laminate film as sheath. A proposal has been made to suppress gas generation by optimizing the carbonic material that is a conductive agent in order to dissolve this problem (see Patent Document 2).    Patent Document 2: JP-A-2005-100770
Patent Document 3 describes on the problem of non-aqueous electrolyte secondary batteries using lithium titanate as negative active material that “It has been found that non-aqueous electrolyte secondary batteries employing lithium titanate and a carbon material respectively as negative active material and conductive agent show poor high temperature characteristics in terms of storage performance and poor cycle characteristics in a high temperature environment because the carbon material and the liquid electrolyte react with each other in the battery in a high temperature environment and a large amount gas is generated. However, this problem does not occur in non-aqueous electrolyte secondary batteries employing a carbon material that occludes and releases lithium for the negative active material. The following finding was obtained as a result of comparing batteries of the two types. When the negative active material is a carbon material, in a charging/discharging cycle, the surface of the carbon material is covered with a film coat. When, on the other hand, the negative active material is lithium titanate, the surface of the lithium titanate and that of the carbonic material are not covered with such a film coat. Therefore, it is assumed that the film coat suppresses the gas generation that arises due to the reaction of the carbon material and the liquid electrolyte. A film coat is formed when the potential of the negative electrode is not higher than the potential of the Li metal by 0.8 V (the potential is relative to the potential of the Li metal hereinafter unless noted otherwise). A particularly high quality film coat is formed when the potential of the negative electrode is not lower than 0.4 V and not higher than 0.5 V. The range of Li occlusion/release potential of a carbon material that occludes and releases lithium is not lower than about 0.1 V and not higher than about 0.9 V and the potential of the negative electrode falls to about 0.1 V in the first charging operation. Therefore, the carbon material and the liquid electrolyte react with each other at a potential of not higher than 0.8 V of the negative electrode to form a film coat, which then stably exists thereafter. On the other hand the range of Li occlusion/release potential of lithium titanate is not lower than about 1.3 V and not higher than about 3.0 V and it is assumed that no film coat is formed. Thus, no film coat is formed on the surface of a negative active material, which may typically be lithium titanate, whose Li occlusion/release potential is higher than the potential of metal lithium by 1 V so that the gas generation due to the reaction of the carbonic material that is a conductive agent and the non-aqueous electrolyte cannot be suppressed.” (paragraphs 0014 through 0017). Therefore, the inventors of the above cited invention obviously was not recognizing that the coat film formed on the surface of the negative electrode when a negative active material that intercalates and deintercalates lithium ions at a potential level of not less than 1.2 V relative to the potential of lithium such as lithium titanate is employed.
Furthermore, the above-cited Patent Document describes that “The inventors paid intensive research efforts to find out that a film coat of a high quality showing excellent ion conductivity is formed on the surface of the negative electrode by providing a negative electrode containing lithium titanate and a carbonic material and a non-aqueous electrolyte containing a chain sulfite and thus a non-aqueous electrolyte secondary battery showing excellent high temperature characteristics and large current characteristics can be realized by using them.” (paragraph 0018) and also shows that the film coat has a carbonate structure (paragraph 0031, 0033, 0114, 0123). However, the above-cited Patent Document does not clearly describe the thickness of the film coat and a non-aqueous electrolyte containing a particular compound of a chain sulfite needs to be used according to the Patent Document.    Patent Document 3: JP-A-2005-317508
Patent Document 4 describes the problem of non-aqueous electrolyte secondary batteries employing lithium titanate as negative active material as follows. “While no problem arises when a non-aqueous electrolyte secondary battery that employs lithium titanate as negative active material is used for the main power source of a portable appliance, a problem of degraded battery characteristics occurs when the non-aqueous electrolyte secondary battery is used as memory backup power source of an operating voltage of about 3.0 V. The reason for this is presumably as follows. When such a non-aqueous electrolyte secondary battery is used as main power source of a portable appliance, the negative electrode is charged to about 0.1 V relative to the potential of metal lithium in the charging process so that a film coat showing excellent ion conductivity is formed on the surface of the negative electrode and the film coat suppresses the reaction of the negative electrode and the non-aqueous electrolyte and hence prevents the non-aqueous liquid electrolyte from being decomposed and the structure of the negative electrode from being destroyed. On the other hand, when a non-aqueous electrolyte secondary battery is used as memory backup power source whose operating potential is about 3.0 V, the charging process proceeds with a minute electric current of about 1 to 5 μA, maintaining a constant voltage state of about 3.0 V for a long period of time so that the negative electrode is charged only to about 0.8 V relative to the potential of metal lithium. Then, no film coat of the above described type is formed on the negative electrode and the negative electrode and the non-aqueous liquid electrolyte react with each other so that the non-aqueous liquid electrolyte is decomposed and the structure of the negative electrode is destroyed.” (see paragraph 0006 and 0007). Thus, while this Patent Document describes that the reaction of the negative electrode and the non-aqueous liquid electrolyte is suppressed in a non-aqueous electrolyte secondary battery employing lithium titanate as negative active material by the film coat formed on the surface of the negative electrode when the negative electrode is charged to about 0.1 V relative to the potential of metal lithium but it does not describe the use of a battery whose negative electrode is charged to about 0.1 V and on which a film coat is formed within a region of potential of the negative electrode higher than 0.8 V relative to the potential of metal lithium. To the contrary, the above-cited Patent Document is based on the fact that no film coat is formed on the surface of the negative electrode of a battery that is operated in a region of potential of the negative electrode higher than 0.8 V relative to the potential of metal lithium. In other words, those skilled in the art can hardly come to an idea of using a battery in which a film coat is formed on the negative electrode in a region of potential of the negative electrode higher than 0.8 relative to the potential of metal lithium because they think that there is a good reason for not using such a battery in such a way.
Additionally, since neither gas generation nor appearance of a swollen battery can be suppressed sufficiently when a battery in which a film coat is formed on the negative electrode is used in a region of potential of the negative electrode higher than 0.2 V relative to the potential of metal lithium as will be described in Examples of this specification, those skilled in the art cannot predict that gas generation and appearance of a swollen battery can be suppressed that by using a battery in which a film coat is formed on the negative electrode in a region of potential of the negative electrode higher than 0.8 V relative to the potential of metal lithium.
Furthermore, the above-cited Patent Document describes that “When a lithium/transition metal composite oxide expressed by LiMnxNiyCozO2 (x+y+z=1, 0≦x≦0.5, 0≦y≦1, 0≦z≦1) is used as positive active material of the positive electrode and the mass ratio of the above described negative active material relative to the positive active material is not smaller than 0.57 and not greater than 0.95, the voltage at the negative electrode at the end of a charging process comes to about 0.8 V relative to the potential of metal lithium so that the non-aqueous liquid electrolyte is prevented from reacting with the negative electrode to become decomposed and the structure of the negative electrode is prevented from being destroyed in a case where the negative electrode is charged while a constant voltage state of about 3.0 V is being maintained . . . ” (paragraph 0022). Thus, since “the voltage at the negative electrode at the end of a charging process comes to about 0.8 V relative to the potential of metal lithium”, no film coat is formed on the surface of the negative electrode as seen from the description of the paragraph 0007 of the above-cited Patent Document and hence those skilled in the art cannot come to an idea of forming a film coat on the surface of the negative electrode to suppress gas generation.    Patent Document 4: JP-A-2005-317509